Cutting tools for chip forming machining of metals, such as round tools, i.e. end mills, drills, etc., and inserts, made of durable materials, such as cemented carbide, cermet, cubic boron nitride or high speed steel, are commonly coated with a surface coating to prolong service life of the cutting tool. It is acknowledged that sufficient hardness of the surface coating is crucial for the wear resistance. The surface coatings are mostly deposited using chemical vapour deposition (CVD) or physical vapour deposition (PVD) techniques.
One of the first wear resistant coatings used was made of titanium nitride (TiN). The high hardness, the high melting point and the oxidation resistance of TiN give a dramatically improved performance and service life as compared to an uncoated cutting tool. These advantageous properties have been further explored and today coatings made of different metal nitrides are used for cutting tools for different applications. By way of example Al is often added to TiN, which gives an improved high temperature oxidation resistance.
It has been realized that the advantageous performance of titanium aluminum nitride (TiAlN) coatings at least partly can be assigned to a miscibility gap in the Ti—Al—N system, which enables stabilization of metastable phases. It has also been discovered that TiAlN coatings exhibit age hardening, i.e. the hardness increases upon heat treatment. The increase in hardness is assigned to the separation of immiscible phases. Cubic TiAlN will decompose upon the heat treatment to cubic TiN and cubic AlN at about 800-900° C., which restrict dislocation motion and gives the age hardening effect. However, at higher temperatures, such as temperatures of about 1000° C., the cubic phase will be followed by a transformation into hexagonal AlN and the coating will dramatically decrease in hardness again which may be detrimental in many applications.
Knutsson et al., Machining performance and decomposition of TiAlN/TiN multilayer coated metal cutting inserts, Surface and Coatings Technology 205 (2011) 4005-4010 describes microstructure characterization and cutting tests of coated cutting tool inserts with PVD Ti0.34Al0.66N/TiN multilayer coatings. The multilayer coatings were found to have enhanced thermal stability and improved mechanical properties as compared to homogeneous coatings of Ti0.34Al0.66N due to a more pronounced temperature induced age hardening effect and over a broader temperature range, up to about 1050° C. This improvement proved to reduce crater wear and flank wear of the cutting tool inserts as compared to the homogeneous coating. It is also disclosed that the hardness increased with decreasing multilayer period.
Although the above age hardening effect improves the thermal stability and performance of coated cutting tools, the dramatic decrease in hardness at higher temperatures remains a problem, in particular in machining of Ni-based alloys, Ti-based alloys and hardened steel.